A small explosion caused minor damage to a U.S. military recruiting center in New York's Times Square area in the early hours of Thursday but there were no injuries, police said.
The U.S. Homeland Security Department said it was investigating whether there was a terrorism link to the explosion.
NY1 television said a small bomb or incendiary device was thrown at the one-story building around 3:45 am New York time, causing a small break in one of the windows.
Police said minor damage was caused to the building's door.
The explosion occurred in the early hours of the morning, when the often bustling area is less busy than at other times.
Asked if there was a link to terrorism in the incident, Homeland Security Department spokeswoman Laura Keehner said "at this time we're still investigating."
Police initially closed off the streets around the busy tourist area, but traffic was allowed through the square three hours after the explosion. Subway trains operations through the busy Times Square station were back to normal, local television said.
New Yorkers are sensitive about such incidents since the hijacked plane attacks destroyed the World Trade Center towers in Manhattan in September 2001.
The targeted building sits in a traffic island between Broadway and Seventh Avenue in Times Square, sometimes called the crossroads of the world.
Capt. Charley Jaquillard, who is in charge of U.S. Army recruiting in Manhattan and works in the center, said nobody was in the building when the explosion occurred.
Asked if he thought it was a terrorist attack, Jaquillard said: "I don't know. Obviously there's some concern, but we'll see what the investigation will determine."
Speaking to Reuters behind the police cordon a block from the building, Jaquillard said the center had been the site of several demonstrations against the U.S. war in Iraq. There had also been cases of vandalism at the center in the past, he said.
Nino Reyes, 26, said he had just opened his coffee and snack stand when he heard an explosion and saw a plume of red smoke shoot up and then turn black.
He said he saw three or four people running away.